Preheat oven to 375F/190C. Choose the smallest possible baking dish that will hold all six peppers standing up, and spread a little olive oil on bottom of pan. Cut off the bottoms of the peppers, cutting off as little as possible to make a flat bottom so the peppers will stand up. (Don't worry if it leaves a small hole.) Slice off the tops of the peppers, cutting off just enough to cut away the stem part. Remove any seeds and membranes from inside the peppers and stand them up in baking dish.

Finely chop the pepper tops and bottoms that you sliced off, as well as the onion. Wash mushrooms if needed, and chop into pieces just slightly larger than you chopped the peppers and onions.

Heat 1 tsp. of oil in a large non-stick frying pan, add onions and peppers, and saute over medium-high heat until they're barely starting to soften, about 4 minutes. Remove to a plate, add 1 tsp. more oil to pan, add chopped mushrooms, and cook until the mushrooms have released their liquid, the liquid has evaporated, and mushrooms are barely starting to brown, about 6 minutes.

Put the sauteed onions and peppers back into the pan with the mushrooms, add the oregano and black pepper to taste, and cook 1 minute more. Stir in the cooked rice and add 1 cup of the crumbled Feta and gently combine. One at a time, hold each pepper over the frying pan and fill with the stuffing, pressing it down as much as you can so it's tightly packed. (We didn't have any extra stuffing, but if you do, just cook it along with the peppers either in the same dish or in a small casserole dish.) When all peppers are stuffed, cover the baking dish with foil (or a lid if it has one) and cook 30 minutes.

After 30 minutes, remove dish from the oven and remove the foil. Put 2 tsp. crumbled Feta on the top of each pepper, pressing it down a little if needed to get it to stick. Bake 20-30 minutes more, until the Feta is starting to melt and lightly brown and peppers are done to your liking. Serve hot and enjoy!